Functionally Gradient Materials Research Articles

Dynamic Analysis of an FGM Beam Undergoing Large Overall Motion in Temperature Field

The dynamic analysis of a functionally gradient material (FGM) beam undergoing large overall motion based on meshless methods is studied in the variable temperature field environment. Considering the high-order terms of nonlinear coupling deformation which were often ignored in previous studies, the high-order rigid–flexible coupled (HOC) dynamic model of a rotating FGM beam with a lumped mass in a temperature field is established by employing Lagrange’s equations of the second kind. Compared with the first-order approximate coupled (FOAC) dynamic model, the HOC dynamic model is more accurate and has a wider scope of application. The validity of the point interpolation (PIM) method and radial point interpolation method (RPIM) in this paper is verified by comparison with simulation results of the assumed mode method (AMM) and the finite element method (FEM). On this basis, the factors affecting the dynamic characteristics of the FGM beam such as the form of temperature field, the functionally gradient index, and the location of added lumped mass are discussed. In this paper, the meshless method with higher computational efficiency and the HOC dynamic model with more accurate results are adopted. This study will provide guidance and reference for dealing with problems in engineering, machinery, aerospace, and other fields in complex environment.

Vibration, Buckling and Stability Analyses of Spinning Bi-Directional Functionally Graded Conical Shells

This paper investigates the free vibration, buckling and dynamic stability of spinning bi-directional functional gradient materials (BDFGMs) conical shells. The material properties vary along the thickness and axial direction. The dynamics model is established based on the first-order shear deformation theory and the governing equations and boundary conditions of the conical shell are derived employing Hamilton’s principle. Subsequently, the differential quadrature (DQ) method is employed to discretize the governing equations into an algebraic system of equations for solving and analyzing the free vibration characteristics of the conical shell. The theoretical model’s accuracy and the solution method’s reliability are rigorously verified. The effects of temperature, functional gradient index, and rotation on the vibration characteristics, traveling wave vibration and critical speed of the conical shell in a thermal environment are systematically explored through numerical analysis. The results indicate that both the material gradient index and temperature increase lead to a decrease in the shell’s natural frequency. For the spinning BDFGMs shell, elevated temperature causes the occurrence of trailing wave vibration to advance to the critical speed. Centrifugal force emerges as the primary factor influencing the critical buckling load and unstable region variation of the spinning shell.

LCA and LCC of wire arc additively manufactured and repaired parts compared to conventional fabrication techniques

Multi-material wire arc additive manufacturing (MM-WAAM) exhibits considerable promise for producing functional gradient materials, components with complex design, and enable a more local supply chain. However, it is still to be determined for which industrial applications MM-WAAM present more benefits than conventional production routes both in terms of environmental and economic impacts. This study undertook a comprehensive cradle-to-grave assessment encompassing two distinct metallic components, manufactured via either additive manufacturing (AM) or conventional fabrication procedures. One of those is a repair case of a forging die, and the other is the production of an industrial tool. Analysis outcomes manifestly revealed pronounced benefits associated with MM-WAAM for these components. For instance, the utilization of MM-WAAM in crafting an optical fiber case injection mould tool yielded energy consumption reduction, attributed to enhanced conformal cooling channels. Additionally, the repair of a forging die via MM-WAAM showcased the potential to curtail new scrap generation during post-processing, prolonging tool lifespan. This study conclusively shows the application-specific nature of MM-WAAM's favourability over traditional processes, thereby offering valuable insights for strategic implementation.

Features for production of dissimilar graded materials using additive manufacturing methods

The article considers the method of obtaining heterogeneous compounds using gradient transitions from intermediate materials. The results of studies of the structure of gradient transitions have shown the possibility of using additive manufacturing methods for obtaining products from heterogeneous metals and for the manufacture of functional gradient materials.

Impact Responses and Wave Dissipation Investigation of a Composite Sandwich Shell Reinforced by Multilayer Negative Poisson's Ratio Viscoelastic Polymer Material Honeycomb.

This analysis investigated the impact wave response and propagation on a composite sandwich shell when subjected to a low-velocity external shock, considering hygrothermal effects. The sandwich shell was crafted using face layers composed of functional gradient metal-ceramic matrix material and a core layer reinforced with negative Poisson's honeycomb. The honeycomb layer consisted of a combination of viscoelastic polymer material and elastic material. The equivalent parameters for the functional gradient material in the face layers were determined using the Mori-Tanaka and Voigt models, and the parameters for the negative Poisson's ratio honeycomb reinforcement core layer were obtained through Gibson's unit cell model. Parameters relevant to a low-velocity impact were derived using a modified Hertz contact law. The internal deformations, strains, and stress of the composite sandwich shell were described based on the higher-order shear deformation theory. The dynamic equilibrium equations were established using Hamilton's principle, and the Galerkin method along with the Newmark direct integration scheme was employed to calculate the shell's response to impact. The validity of the analysis was confirmed through a comparison with published literature. This investigation showed that a multilayer negative Poisson's ratio viscoelastic polymer material honeycomb-cored structure can dissipate impact wave energy swiftly and suppress shock effectively.

Experimental investigation of compressive behaviors of functionally graded composites material based on engineering cementitious composites and normal concrete

A new kind of functional gradient material (FGM) based on engineering cementitious composites (ECC) and normal concrete (NC) was proposed in this study. This material offers enhanced crack control while minimizing ECC consumption. Foremost, uniaxial compression tests were carried out on single-layer NC-ECC composites featuring varying ECC volume fractions (0%, 20%, 40%, 60%, 80%, and 100%). These tests yield an elastic modulus formula, E(y), for the NC-ECC composite. Subsequently, based on the E(y) formula, a 5-layer NC-ECC FGM is designed, with each sub-layer having distinct ECC volume fractions. This is contrasted with pure ECC and 2-layer NC-ECC specimens for reference. The study concludes with an analysis of the failure patterns, load-displacement curves, and elastic moduli of the NC-ECC FGM. The results demonstrate consistent performance trends of elastic modulus in single-layer NC-ECC composite specimens, confirming the feasibility of employing a functional gradient between NC and ECC. Utilizing the formula E(y) derived from single-layer NC-ECC, the ECC volume fractions in the 5-layer NC-ECC FGM are computed as 100%, 86.89%, 68.13%, 41.55%, and 0% for each layer. Importantly, the NC-ECC FGM exhibits a failure mode resembling ECC specimens, with primary cracks not extending along interfacial layers. In contrast, 2-layer NC-ECC specimens experience cracking along interfacial boundaries. The compressive strength of NC-ECC FGM surpasses that of ECC specimens by 1.42 times while retaining ECC-like ductility, showcasing a 25.85% improvement over 2-layer NC-ECC. The findings from this study provide valuable insights for the design and subsequent engineering applications of functional gradient materials.

Progress of ceramic materials in the application of armor protection

Bulletproof ceramics have emerged as crucial elements within contemporary ballistic protection systems, aiding in safeguarding individuals and assets against various threats. As the demand for enhanced protection continues to rise, understanding the evolution and advancements in ceramic armor materials becomes imperative. This paper presents a comprehensive analysis of the evolution, properties, and advancements in ceramic armor materials. The nuanced comparative study of these ceramics includes highlighting the distinct ballistic properties of each, with a particular emphasis on the balance between hardness and potential fragility, as well as Silicon Carbide’s promising composite ceramics. The paper spotlights the transformative potential of graphene-modified ceramics, functional gradient materials, and micro-laminates. Alumina ceramics underscore the significance of microstructural optimization and the role of grain size adjustments. Conclusively, this paper offers a panoramic view of the past, present, and future trajectories of ceramic armor materials, advocating for continued research and innovation in this critical domain.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review.

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Effects of TiB2 content and gradient structure on microstructural characterization and mechanical properties of TiB2/Al7050 composites

TiB2 particle reinforced Al7050 matrix homogeneous composites (HC), TiB2/Al7050 three-layer conventional linear gradient functionally gradient materials (TFGM), and TiB2/Al7050 five-layer functionally gradient materials with a bionic structure (FFGM) were successfully prepared using vacuum hot-pressed sintering (VHS) method. The effects of gradient structure and TiB2 particle content on the microstructural characterization and mechanical properties of these composites were investigated. The results demonstrate that the bending strength of all composites improves with the increase in TiB2 content. Comparatively, the bending strength of functionally gradient materials (FGM) are generally superior due to a combination of the interlayer strength of each layer, heterogeneous deformation-induced strengthening, strength-plasticity synergy of the whole material, and synergistic interlayer strengthening under strong bonding. Notably, the enhancements in bending strength and strain exhibited by FFGM are more pronounced, highlighting its "soft-hard interleaved" and "strong-tough complementary" structural pattern. The research provides valuable insights for the application and further development of FGM.

Microstructure and mechanical properties of functional gradient materials of high entropy alloys prepared by direct energy deposition

In this study, FeCrCoNiMo0.5W0.75 HEAs functional gradient materials were prepared by the direct energy deposition (DED) technique, and the phase transition, microstructure and mechanical properties of the materials along the building direction were investigated. The material realizes a transition from a single FCC phase to an FCC + TCP phase from the bottom to the top. The bottom and middle of the material are typical dendritic (DR)-interdendritic (ID) structures, and at the top, due to the precipitation of the eutectic structure σ phase and μ phase, the material transforms into a hypoeutectic structure of FCC + eutectic. According to the EBSD results, the interior of the material is mainly columnar crystals that grow vertically to the molten pool. This unidirectional columnar growth makes the material have a strong texture in the <100> direction. In addition, a large degree of grain refinement is exhibited from the bottom to the top, with the grain size reduced from 19 μm to 6.5 μm. According to the TEM results, the density of dislocations is much higher near the phase and grain boundaries than in other regions, and dislocation entanglement is formed by the continuous accumulation of loose dislocations. The micro-hardness of the material increased along the construction direction up to 892.5HV0.5, and the material has good compressive strength up to 2041 ± 45 MPa. The material has excellent wear resistance and high-temperature wear resistance, forming a continuous oxide layer at 800 °C, which greatly reduces the friction coefficient and wear rate.

Impact Resistance and Energy Absorption Characteristics of the Bionic Turtle Shell Structure

The microstructure of the shell is a kind of “sandwich” structure with a denser outer layer and a less dense inner layer, density, which can be identified as a functional gradient material in engineering. It has excellent performance, such as earthquake resistance and shock resistance. In order to study the reliability of this kind of material, a kind of shell structure is designed in this paper. In the range of elastic deformation constraints of the material, simulate and calculate bionic structure the bionic structure. Firstly, a three-point bending analysis is carried out to investigate the mechanical properties of the bionic structure under a bending load. Secondly, through the Finite Element Analysis, the mechanical properties of the shell structure and the general structure under the same load are analyzed and compared. The simulation results show that in the three-point bending analysis, the maximum equivalent stress of the simulated shell structure under impact load is reduced by 1.4 times compared with the general structure. The energy absorption analysis showed a 1.6-fold increase.

In-situ observation of fracture behaviors in TiB2p/Al7050 composites with different gradient structure

The study employed the vacuum hot-pressed sintering method to fabricate TiB2p/Al7050 homogeneous composites (HC) and TiB2p/Al7050 functionally gradient materials (FGM), including three-layer conventional FGM (TFGM) and five-layer bionic interleaved FGM (FFGM). The crack-initiation fracture toughness (KIc), crack-propagation fracture toughness (KJc), and crack propagation performance were investigated to examine the impact of gradient structure and TiB2p content. The results highlighted the crucial role of TiB2p content in determining the fracture toughness of the three composites. The fracture toughness of HC requires consideration of the synergistic effects of strength and plasticity, while that of FGM is influenced by the combined effects of strength, plasticity, and interlayer synergistic factors. Additionally, the in-situ observation of the crack propagation path revealed a toughening mechanism that contributes to the improved fracture toughness of these composites. This mechanism includes crack deflection, branching, plastic deformation of the matrix, and crack blunting, which consumes the energy required for crack propagation, reduces the stress concentration at the main crack tip, and delays crack propagation. It is worth mentioning that among these composite materials, FFGM demonstrates the highest fracture toughness, with KIc and KJc increasing up to 22.39% and 22.23% above the average, respectively. The excellent performance can be attributed to its highly effective toughening mechanism. Overall, the study provides valuable insights into the design and development of FGM with superior fracture resistance. Hence, these findings can support the development of advanced materials with improved toughness and potential applications in various fields.

Effect of Convective Cooling on the Temperature in a Friction System with Functionally Graded Strip.

An exact solution of the boundary-value problem of heat conduction was obtained with consideration of heat generation due to friction and convective cooling for the strip/semi-space system. Analytical solutions to this problem are known for the case with both friction elements made of homogeneous materials or a composite layer with a micro-periodic structure. However, in this study, the strip is made of a two-component functionally gradient material (FGM). In addition, the exact, asymptotic solutions were also determined at small and large values of the Fourier number. By means of Duhamel's theorem, it was shown that the developed solution for a constant friction power allows to obtain appropriate solutions with a changing time profile of this value during heating. Numerical analysis in dimensionless form was carried out for the FGM (ZrO2-Ti-6Al-4V) strip in combination with the cast iron semi-space. The influence of the convective cooling intensity (Biot number) on the temperature field in the considered friction system was investigated. The developed mathematical model allows for a quick estimation of the maximum temperature of systems, in which one of the elements (FGM strip) is heated on the friction surface and cooled by convection on the free surface.

Effect of composition gradient design on microstructure and mechanical properties of dual-wire plasma arc additively manufactured 316L/IN625 functionally graded materials

Wire and arc additive manufacturing (WAAM) is a promising technology for the preparation of refractory metal functional gradient materials (FGMs) due to its high material utilization, high deposition efficiency and ability to create large-sized parts. In the present work, 316L/IN625 FGM with chemical composition gradients of 25 wt% and 100 wt% were fabricated using dual-wire plasma arc additive manufacturing (DW-PAAM) by changing the wire feeding speeds (WFSs) layer by layer. The phase evolution, microstructure, chemical composition and mechanical properties of the FGM were analyzed. The results demonstrated that the microstructure in all regions was predominantly the austenite phase, and no cracks or defects were observed. The microstructure of the 100 wt% 316L zone was composed of the austenite phase (face-centered cubic, FCC) and a small amount of ferrite phase (body-centered cubic, BCC), whereas mainly FCC structure was observed in the other chemical composition regions. There was a certain error between the actual composition gradient and the designed gradient. The microhardness transition of S1 was smoother and wider, while the tensile test results of S2 were better. In addition, the position of the tensile fracture was different for S1 and S2. In conclusion, the DW-PAAM process shows great potential for manufacturing FGM with desirable properties for various industrial applications. Further studies are warranted to optimize process parameters and develop new design strategies to enhance the performance of FGM in various industrial applications.

Stiffness and hardness gradients obtained by laser surface treatment of aged β-Ti alloys: The role of ω phase

Functional gradient materials are interesting for optimizing the properties of engineering parts. Laser surface treatment makes it possible to obtain a stiffness gradient on titanium parts; such graded material is attractive due to the potential for orthopedic implant performance improvement and other applications as well. This work used laser surface treatment without fusion on two aged titanium alloys: Ti8Mo and Ti30Nb1Fe. This processing route aimed to fully dissolve the α-phase precipitates in a surface layer and, consequently, reduce the elastic modulus, producing a stiffness gradient from the surface to the inner material. Firstly, single tracks were obtained for different laser powers, kept constant the other parameters, aiming to determine the optimum condition to avoid surface fusion and produce the thickest-possible modified layer. Then, multi-track overlapping produced a modified continuous layer. The aged titanium alloys were characterized by optical and scanning electron microscopy and X-ray diffraction. The single tracks were characterized by stereoscopic and optical microscopy. Transmission electron microscopy and instrumented indentation completed the characterization of the laser-modified layer. Laser surface treatment produced a less-rigid surface layer in the Ti8Mo alloy due to the obtainment of β and α″ titanium phases with lower elastic modulus. The surface treatment did not change the material's hardness in this case. On the other hand, the precipitation of the ω phase during surface cooling of the Ti-30Nb-1Fe alloy prevented the elastic modulus from decreasing and increased the modified-layer hardness. Hence, the absence of ω phase precipitation is a condition to obtain a less-rigid surface layer by laser surface treatment on aged titanium alloys.

Copper-nickel functionally magnetic gradient material fabricated via directed energy deposition

In this paper, a functional gradient material (FGM) of CuSn10 and Inconel 718 is fabricated through a laser-based directed energy deposition (DED) additive manufacturing process. Experimental studies are performed to examine the microstructures, the interface bonding behaviors, and the physical and mechanical properties of the resulting FGMs. The result shows that a sound metallurgical bonding was formed at the interface section. The electrical resistivity for CuSn10, IN718, and CuSn10 – IN718 samples are tested for a temperature range of 320 to 390 K, CuSn10 – IN718 specimen displayed its value in the middle range of its counterparts. The CuSn10 – IN718 FGM displayed the highest absolute Seebeck coefficient in the temperature range 320 K to 390 K. The change in the Seebeck coefficient of the FGM showed a 70 % increase from the single material specimens. This boost indicated a reduction in carrier density across the interface, theorized to be influenced by the interface of the FGM. During the compression strength test, a folding deformation occurred in the copper alloy region but neither buckling nor deformation was triggered at the interface section.

Studies on the 316/NiTi functionally gradient ultra-thick coatings fabricated with directed energy deposition: Microstructure, crystallography and wear mechanism

316 and NiTi alloy powders were selected as raw materials to prepare functional gradient materials (FGM) by Directed energy deposition (DED) on 316 substrates due to the wide range of engineering and industrial applications. The 316/NiTi FGM with NiTi composition gradually transformation along the building direction was prepared, and the relationship between microstructure and wear properties was revealed through a series of microstructure and performance experiments. The phase composition of 316/NiTi FGM mainly consists of intermetallic compounds composed of Fe, Ni, and Ti, while the bonding morphology between the gradient layers exhibits typical dendrite growth. The overall chemical composition distribution of 316/NiTi FGM showed a gradient transformation without abrupt variation confirmed by qualitative and quantitative analysis. From the bottom to the top, the preferred orientation of FGM grains changes from 〈001〉 to random orientation, benefiting from the cooling effect of the matrix and surrounding environment on FGM leading to the transformation of the maximum temperature gradient. The FGM demonstrates better wear resistance at both low- and high-speeds compared to 316 substrates, which is due to the superelasticity and high hardness of intermetallic compounds resulting in slighter oxidative and abrasive wear.

Improved ballistic performance of a continuous-gradient B4C/Al composite inspired by nacre

Functionally gradient materials (FGMs) primarily composed of a discretely layered structure with relatively sharp macroscopic interfaces are highly susceptible to interlayer failure under impact loading. Nacre is regarded as a model system for lightweight armor with high impact-resistant performance owing to its multiple length-scale and gradient structures. However, the simultaneous translation of both structural features into artificial assemblies remains challenging. Herein, we constructed new B4C/2024Al FGMs with eliminated sharp interfaces, which reproduced the hierarchical and gradient architectures of nacre. A combined experimental and numerical approach was employed to investigate the protective mechanisms of nacre-like FGMs subjected to 7.62 mm armor-piercing incendiary bullet, and two homogeneous composites were also tested under the same conditions for comparison purposes. The FGMs exhibited improved performance to resist ballistic attacks and maintain integrity compared with the uniform structures. As the ceramic contents decreased along the thickness, the energy dissipation mechanisms transitioned from eroding projectile to plastic deformation. The toughness gradient and varying energy dissipation modes prevented the formation of crushed cones at the ceramic-rich layers. The nacre-like structure created several extrinsic toughening mechanisms, which led to the increased energy-absorbing capability of the target. Meanwhile, the eliminated abrupt interfaces avoided the occurrence of interlayer tearing by diminishing the tensile wave strength and the sudden change of wave impedance, which prolonged the projectile-target interaction period and strengthened the eroding effect on the projectile. The present study opens new opportunities to design advanced lightweight FGMs armors with high impact-resistant protective performance.

Mathematical Model of Properties and Experimental Fatigue Investigation at Elevated Temperatures of Functionally Gradient Materials

Fatigue that occurs at elevated temperatures is called thermal fatigue. High temperatures and cycling loads cause thermal fatigue that can cause component failures. In this paper, induced the mathematical models for functionally gradient materials and three models of functionally gradient materials (FGMs) manufactured by permanent casting were tested to predict their thermal fatigue life. FGMs have been tested to determine the effect of fatigue and temperature interactions. FGMs models were of FGM1, FGM2, and FGM3 respectively, with the following volume fractions and gradations: [100%Al-50%Al50%Zn-50%Zn50%Al-100%Zn], [100%Al-30%Zn70%Al-70%Zn30%Al-100%Zn] and [100%Al-70%Zn30%Al-30%Zn70%Al-100%Zn]. The experimental procedure presented the mechanical properties as modulus of elasticity at two levels of temperatures (80 oC, and 160 oC). Through the results of the tensile test at high temperatures, it was noted that the reduction percentage was high in the second type. This was especially for the yield strength value, where the percentage reached 43% at 160 °C. In the second type, the ultimate strength was affected more at 160 °C: the decrease percentage reached 33%, and the elastic modulus at the same temperature fell by 32%. The third type is the least affected at high temperatures, as the percentage of properties decrease at 160 °C reached 16, 17, and 16% for modulus of elasticity, ultimate strength, and yield strength, respectively. Fatigue strength is the most significant among the mechanical properties, where the highest value of the fatigue limit for the third type was experimentally 149.68 MPa. While the lowest value of fatigue strength was the second type, and at the same time, on the contrary, it was less affected at high temperatures, with a rate of 8%, which reached the decrease. The simulation results from the ANSYS program regarding fatigue at high temperatures were acceptable and gave reasonable variation ratios and were very close to the experimental results.

Design and Geometric Characterization of Three-Dimensional Gradient Heterogeneous Bone Tissue Structures Based on Voronoi

Bone tissue is one of the most transplanted tissues. During transplantation, additive manufacturing enables fast and efficient production and fabrication of shaped external implants, and designing porous interconnected bionic external implants is the focus and difficulty of bone tissue engineering. Based on Voronoi geometry to design a bone tissue structure with hole position gradient—Voronoi gradient structure (VFGM, Voronoi functional gradient materials), the design hole center conforms to the uniform gradient probability spatial distribution, and the boundary in 3D Voronoi is calculated with the hole center as the bone trabecular skeletal. It can be used to design a variety of heterogeneous structures to match with the damaged location. The heteromorphic VFGM structure is highly similar to the human bone structure and can provide geometric design for bone tissue engineering external implants.